The present invention is directed to general-purpose, pressure-sensitive adhesives which uniquely enhance the rate of conversion of label stock to matrix-stripped label stock.
With reference to attached FIGS. 2, 3 and 4, matrix-stripped label stock is conventionally produced by pressure-sensitive-adhesive label stock 10, comprised of a face material 12, a pressure-sensitive-adhesive layer 14, and a release liner 16, usually silicon-coated, being passed between driven-anvil roll 18 and driven-die roll 20, having cutting edges 22, with penetration of the die into the laminate being determined by breaker 24. Labels 26 are cut to the release liner. There is removed from the laminate a matrix web 28 which, as illustrated in FIG. 4, bears the outline of the labels cut. The cut matrix web, which is waste, is wound for disposal.
Difficulty in removing the matrix web is dependent upon label configuration as well as the adhesive, employed. Conventionally, a label is cut from stock 61/2 inches or 151/2 inches wide. Some of the more standard dies are Die A, which forms "file folder" labels, illustrated in FIG. 5, and Die B, illustrated in FIG. 6. Labels with longitudinal matrices, such as those illustrated in FIG. 7, or those with many sharp corners or points, such as the "starburst" label illustrated in FIG. 8, are parcularly difficult to die-cut and matrix-strip at high speed.
Presses used for the die-cutting and the matrix-stripping include Webtron, which has an operating speed of up to 650 feet per minute, and the Mark Andy press, which processes stock 151/2 inches wide, and which has a an operating speed of up to 1,000 feet per minute.
As previously indicated, although die configurations have limited the rate at which the converter can be operated, the adhesives also play a significant part in this limitation. Many are hot-melt adhesives.
The majority of the commercial hot-melt, pressure-sensitive adhesives used today are based on a thermoplastic rubber, normally a block copolymer such as styrene-isoprene-styrene or styrene-butadiene-styrene, conventionally sold as Kraton.RTM. rubbers by Shell Chemical Co. Other major components are tackifying resins, used with or without plasticizing oils. Minor components include antioxidants, fillers and the like. The concentration of thermoplastic rubber present in conventional hot-melt adhesives ranges from about 25 to about 55 percent by weight of the total weight of the composition.
For a given adhesive it has been expected that convertibility will vary with matrix-die complexity. For a given die, as machine speed is increased, the first sign of failure of operation is a phenomenon known as matrix flagging, where the matrix segments normal to the machine direction lift and tear. Generally, this is the highest practical operating speed. Higher speeds can result in the matrix remaining with the face material, due to tearing or the like, and in more aggravated cases, the labels will lift with the matrix because the adhesive has not been properly cut, despite precise configurations of the die to ensure penetration through the adhesive.
Since the die is operated in cooperation with the waste-removal system, the overall system has been limited by the rate at which the waste matrix can be removed.
We have sought to develop an adhesive system which is adaptive to conventional methods of label manufacture by providing overall good adhesive properties but which enables the speed of conversion to be increased.